It is known to position a generator plate in an air flow to generate Karman vortices. Detection of the frequency of these vortices can be used as an indication of the mass of air flowing by the generator plate. For example, U.S. Pat. No. 4,312,237 issued Jan. 26, 1982 discloses various embodiments of narrow, elongated generator plates used for the generation of Karman vortices in a fluid stream. In one embodiment of U.S. Pat. No. 4,312,237, a generator plate has a shedding section wherein an aperture intersects side surfaces of the generator plate to form a leading aperture edge which serves as a shedding corner for the generation of vortices.
Also, U.S. Pat. No. 4,312,236 issued Jan. 26, 1982 discloses a vortex generating device having a narrow generator plate which is immersed in a fluid flow to generate Karman vortices. The generator plate has side surfaces parallel to the direction of fluid flow. The side surfaces have a shedding region therein forming shedding corners at which vortices detach from the generator plate.
Air flow meters using such vortex generators have produced stable vortices over a wide range of fluid flow rates. Such devices have functioned particularly well and operated in accordance with the acoustical techniques disclosed in U.S. Pat. No. 3,680,375 to Joy et al.
However, there has been a problem of adjusting the vortex frequency to a precise value at a given flow for flow meters that do not have precise mechanical dimensions.
The vortex flowmeter has the characteristic of having a quasi-digital output where each output pulse represents the passage of a certain amount of fluid. This amount of fluid is determined by the flow duct area and the cross section width of the vortex generator. This relationship is defined by: EQU f=SQ/dA Equation 1
where
f is the vortex frequency PA1 S is the Strouhal constant PA1 Q is the volumetric flow rate PA1 d is the vortex generator width and PA1 A is the flow duct area
In order to produce flowmeters having an identical vortex frequency to fluid flow relation, the mechanical duct area must be held to precise tolerances.
For some applications where tight tolerances cause a cost penalty, such as in the automotive field, a means is needed to adjust some mechanical dimension to change the frequency-to-flow relation in order to offset manufacturing tolerances.